The aim of this study is to determine the role of N-methyl-D-aspartate (NMDA) receptors in the development and maintenance of neuropathic pain, and to specify how the sympathetic nervous system interacts with NMDA receptors to drive hyperalgesia. The investigators hypothesize that neuropathic pain is initiated by prolonged or supranormal activation of NMDA receptors in the spinal cord, and as a consequence these neurons cannot reverse their hyperalgesic state or are driven by a secondary system. The last 2 subjects have completed the study.